Implement indexing events to Elasticsearch using IndexedRecord

[rikukissa]

Description

Implement a indexing function for event. All events should always have only one Elasticsearch entry, so id should always be unique in the index.

It is important to implement one pure function with a signature of

function eventToEventIndex(event: Event, eventConfig: EventConfig): EventIndex

As this function will be later on used to implement a full reindex. For this purpose, it's useful to write the side-effectful write function as function indexEvents(events: EventIndex[]) from the start